1. Field of Invention
2-Substituted malondialdehyde compounds are useful as co-developers in combination with hindered phenol developers to produce very high contrast black-and-white photothermographic and thermographic elements.
2. Background of the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) which are developed with heat, without liquid development have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a relatively non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example for the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.
The photosensitive compound is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those silver atoms are able to catalyze the reduction of the reducible silver source. It has long been understood that silver; atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. The silver halide may be made "in situ," for example by adding a halogen-containing source to the reducible silver source to achieve partial metathesis (See, for example, U.S. Pat. No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049). The silver halide may also be made "ex situ" (i.e., be pre-formed) and added to the organic silver salt. The addition of silver halide grains to photothermographic materials is described in Research Disclosure, June 1978, Item No. 17029. It is also reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely, so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further developed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms (i.e., the latent image). This produces a black-and-white image. In photographic elements, the silver halide is reduced to form the black-and-white image. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white image while much of the silver halide remains as silver halide and is not reduced.
In photothermographic elements the reducing agent for the organic silver salt, often referred to as a "developer," may be any material, preferably any organic material, that can reduce silver ion to metallic silver. At elevated temperatures, in the presence of the latent image, the silver ion of the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and cause fog during preparation and coating of photothermographic elements. As a result, hindered phenol reducing agents have traditionally been preferred.
Thermographic imaging constructions (i.e., heat-developable materials) processed with heat, and without liquid development, are widely known in the imaging arts and rely on the use of heat to help produce an image. These elements generally comprise a support or substrate (such as paper, plastics, metals, glass, and the like) having coated thereon: (a) a thermally-sensitive, reducible silver source; (b) a reducing agent for the thermally-sensitive, reducible silver source (i.e., a developer); and (c) a binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, ascorbic acid derivatives, and the like, whereby an image of elemental silver is formed.
Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such instances, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of about 60.degree.-225.degree. C., resulting in the formation of an image.
The imaging arts have long recognized that the fields of photothermography and thermography are dearly distinct from that of photography. Photothermographic and thermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing. See for example the discussion in U.S. patent application Ser. Nos. 08/530,066 and 08/530,694 both filed Sep. 19, 1995.
U.S. Pat. No. 5,496,695 describes hydrazide compounds useful as co-developers for black-and-white photothermographic and thermographic elements. These elements contain (i) a hindered phenol developer, and (ii) a trityl hydrazide or a formyl phenylhydrazine co-developer, and provide elements having high Dmax (&gt;5.00), fast photospeeds, and high contrast (&gt;20.0).
U.S. patent application Ser. No. 08/529,982 (filed Sep. 19, 1995) describes combinations of hindered phenol developers with acrylonitrile compounds as co-developers for black-and-white photothermographic and thermographic elements. A trityl hydrazide or a formyl phenylhydrazine co-developer may also be included.
U.S. patent application Ser. No. 08/530,024 (filed Sep. 19, 1995) describes combinations of hindered phenol developers, a trityl hydrazide or a formyl phenylhydrazine, and amine compounds as co-developers for black-and-white photothermographic and thermographic elements.
U.S. patent application Ser. No. 08/530,066 (filed Sep. 19, 1995) describes combinations of hindered phenol developers, a trityl hydrazide or a formyl phenylhydrazine, and hydrogen atom donor compounds as co-developers for black-and-white photothermographic and thermographic elements.
U.S. patent application Ser. No. 08/530,694 (filed Sep. 19, 1995) describes combinations of hindered phenol developers, a trityl hydrazide or a formyl phenylhydrazine, and hydroxamic acid compounds as co-developers for black-and-white photothermographic and thermographic elements.
It would be especially desirable to be able to achieve in dry photothermographic or thermographic elements the high contrast that is currently available in wet silver halide materials. It would be advantageous to improve the reactivity of these dry systems, allow the reduction in the amount of silver by lowering the silver coating weights, reduce the amount of developer and co-developer compounds needed to achieve high contrast, and lower costs. New developing agent systems for use in photothermographic and thermographic elements are therefore desired.